Identification of sources of ions in early monsoon precipitation over Kolkata Metropolis and two adjoining towns

Abstract

Sources of cations and anions present in early monsoon precipitation (rain) were identified at one location each in Kolkata city (urban), in Baidyabati (peri-urban) and Rishra (urban-industrial) towns by enrichment factor and Principal Component Analysis. Ingress of transboundary air into local atmosphere was assessed by back trajectory analysis of air masses to assess the role of regional air masses on local air quality. Average rainwater pH in Kolkata and Baidyabati were close to 6.7 but in Rishra, average pH was 6.1 and an acid rainfall event was recorded. Ammonia (NH4+) was the principal acid neutralizing cation, followed by Ca2+, Mg2+ and K+. Bicarbonate (HCO3−) was the dominating anion while SO42− was also prominent. Crustal, marine sources and local emissions were identified to have contributed heavily to ions detected in rainwater. Air trajectory analysis revealed ingress of maritime air and air flowing over land masses to study sites, hinting to its role in influencing local atmospheric chemistry. Contribution of anthropogenic sources towards SO42− and NO3− ions was predominant (>93% and > 97%, respectively) followed by marine and crustal sources. Factor analysis revealed the role of 3 major factors in governing rainwater chemistry, accounting for >90% of the total variance at all three sites.